The invention relates generally to metal lenses for increasing bandwidth while reducing length in a horn antenna.
The metal lens antenna was described by W. E. Kock in “Metal Lens Antennas,” Proceedings of the I.R.E. and Waves and Electrons, November 1946 (http://www.qsl.net/n1bwt/chap3.pdf). The metal lens utilized a series of parallel plate waveguides that were shaped on the front or rear to give a concave or convex appearance. As a ray of electromagnetic radiation traverses the parallel plates at different points, the different phase velocity of the wave within the plates would cause a beam focusing or expanding effect. Such a metal lens is compared to a typical lens fabricated of dielectric materials.
The metal lens suffers from small bandwidth caused by several phenomena. The first is due to chromatic aberrations, and Kock described a stepped lens approach to help correct this error. The second is due to impedance match at the input of the lens. Kock described a technique to tilt the lens from the horn axis to help minimize the reflected energy back into the horn's input. However, this method can distort the phase of the wave through the lens and has limiting effect.